An imaging device, or sensor, is a photosensitive electronic component serving to convert electromagnetic radiation into an analog electrical signal. This signal is thereafter amplified and then digitized by an analog-digital converter and finally processed to obtain a digital image. The imaging device exploits the photoelectric effect, which allows the incident photons to tear electrons from each active element. An imaging device generally comprises photosites arranged as a matrix, each photosite corresponding to a pixel of an image.
A photosite comprises at least one photosensitive zone, for example, a photodiode, and one zone for reading the charge accumulated in the photodiode. The photons captured by the photodiode are converted into electron/hole pairs stored in the photosite before being read by an electronic system.
Generally, this electronic system, which controls the photodiode, comprises, especially when the photodiode is a fully depleted photodiode, a transfer transistor permitting the transfer of charge from the photodiode to a charge reading zone. This reading zone forms a measurement node (or “sensing node”), which is connected to typical reading electronics comprising a reading transistor.
A photodiode operates according to a cycle comprising at least one integration step, one measurement step, and one re-initialization step. The integration step includes the photogeneration of charge and its accumulation during exposure of the photodiode to light. The measurement step includes the generation of a signal dependent on the quantity of photogenerated charge accumulated in the photodiode. The re-initialization step includes the elimination of this charge.
Imaging devices are generally used for digital cameras intended for the general public, but they can also be used in security or monitoring devices, especially in onboard security devices embedded aboard motor vehicles. Imaging devices make it possible to detect, for example, obstacles, etc. Such security devices must be able to operate continuously in an optimal manner, and consequently, continuously check their various elements.